110 JUNIOR GRADE SCIENCE 



Tin. Tin is a brittle white metal, about seven times as dense as 

 water. It is malleable and in sheets is familiar as " silver paper." The 

 noise which is heard when tin is bent is due to its crystalline structure. 

 The individual crystals rubbing against one another during bending 

 cause the characteristic sound known as the " cry of tin." 



On account of the fact that it does not tarnish easily, tin is used for 

 coating other metals, such as iron. When heated it increases in weight 

 and changes into a greyish powder. 



52. EXAMINATION OF PLATINUM. 



i. Describe the physical properties of the piece of platinum foil supplied, 

 but be careful not to injure it in any way, as it is more costly than gold. 



ii. Effect of heat. Hold the platinum foil in the Bunsen flame and make 

 observations. Allow it to cool. Has the metal changed in any way ? 



Platinum. Platinum is a white metal rather like silver in appear- 

 ance. It has a density of 21 '5, or nearly twice that of lead. When 

 heated in a Bunsen flame it does not melt, and on cooling is found to 

 be quite unchanged. This property makes it most valuable for making 

 certain laboratory apparatus, such as crucibles which have to withstand 

 high temperatures. 



Its rate of expansion when heated is the same as that of glass, and 

 hence it can be fused into glass vessels without fear of their cracking 

 on cooling. As platinum is a good conductor of electricity, this pro- 

 perty is used in the making of electric light bulbs. 



53. EXAMINATION OF MAGNESIUM. 



i. Describe the physical properties of the piece of magnesium ribbon 



supplied. 



ii. Effect of heat. Hold the magnesium 

 ribbon by means of a tongs in the flame. 

 Make observations. 



iii. Change of weight when magnesium 

 is burnt. Weigh a crucible and its lid 

 with a piece of magnesium, which, folded 

 lightly, is placed in the crucible. Heat it 

 strongly in a burner, taking care to let no 

 fumes escape (Fig. 85). To do this, keep 

 on the lid, and only raise it a little when 

 the flame is removed. The magnesium is 

 seen to burn brightly in places, but if 

 care is taken, no fumes are lost. When 

 finished, the mass should be in the form 

 FIQ. 85,-How to heat magnesium of wh * te P<?er. Allow to cool, and 

 powder or ribbon in a crucible. weigh the crucible with the lid and 



